The present invention relates to an electric discharge machining apparatus, and more particularly to prevention of the occurrence of radio interference due to electromagnetic waves which are generated during an electric discharge machining operation.
FIG. 1 is a sectional view showing a conventional electric discharge mmachining apparatus as disclosed, for instance, by Japanese Patent Application (OPI) No. 15300/1983 (the term "OPI" as used herein means "unexamined published application"). In FIG. 1, reference numeral 1 designates a machining reservoir; 2, a machining level block on the bottom of the machining reservoir 1; 3, a workpiece on the machining level block 2; 4, a machining electrode held above the workpiece 3 (hereinafter referred to merely as "an electrode", when applicable); 5, a machining solution in the machining reservoir 1; 6, a machining power unit provided outside the machining reservoir 1; 7, electric power supplying feeder lines through which the machining power unit is connected to the machining reservoir 1 and the electrode 4; and 8, a shield room housing the machining reservoir 1 and the machining power unit 6, the shield room being grounded.
In the conventional electric discharge machining apparatus thus constructed, the workpiece 3 and the electrode 4 are disposed in the machining solution with a small gap therebetween, and a pulse voltage is applied across the workpiece 3 and the electrode 4 through the feeder lines 7 by the power unit 6. As a result, electric discharge occurs between the workpiece 3 and the electrode 4; that is, machining current flows in a closed loop consisting of one of the feeder lines 7, the electrode 4, the workpiece 3 and the other feeder line 7, thus machining the latter 3. When the electric discharge occurs, or the pulse voltage rises or falls, electromagnetic waves are radiated from the gap between the workpiece 3 and the electrode 4, or from the electric power supplying feeder lines 7. If there is, for instance, an NC machine tool, or a signal receiving device such as a radio set or television set around the electric discharge machining apparatus, the electromagnetic waves may cause the NC machine tool to operate erroneously or cause noises in the signal receiving device; that is, they may cause radio interferences. However, as was described above, the machining reservoir 1 containing the workpiece 3 and the electrode 4 and the power unit 6 are housed in the shield room 8, and therefore the electromagnetic waves radiated from the gap between the workpiece 3 and the electrode 4, or from the feeder lines 7, are shielded by the shield room 8, so as not to leak outside. That is, the radio interference by the electromagnetic waves is prevented.
FIG. 2 is a sectional diagram showing another example of a conventional electric discharge machining apparatus which has been disclosed by Japanese Patent Application (OPI) Nos. 69301-1979, 69302/1979 and 293722/1986. In FIG. 2, parts corresponding functionally to those already described with reference to FIG. 1 are designated by the same reference numerals. Further in FIG. 2, reference numeral 9 designates an electrode servo shaft for driving the electrode 4; and 10, an electrically conductive cover laid over the machining reservoir 1 in such a manner that the electrode servo shaft 9 penetrates the cover 10.
In the conventional electric discharge machining apparatus shown in FIG. 2, electromagnetic waves radiated from the gap between the workpiece 3 and the electrode 4 is shielded by the electrically conductive cover 10, so that they may not leak outside the machining reservoir 1; that is, radio interference due to the electromagnetic waves is prevented.
In the first example of the conventional electric discharge machining apparatus, the machining reservoir 1 and the machining power unit 6 are housed in the shield room 8. Therefore, the electric discharge machining apparatus is disadvantageous in that installation of the shield room 8 may occupy a relatively large space and the installation must be carried out by a person skilled in the art, with the result that the electric discharge machining apparatus is high in running cost.
In the second example of the conventional electric discharge machining apparatus, the machining reservoir 1 is covered with the electrically conductive cover 10 which is penetrated by the electrode servo shaft 9 with the electrode 4. Therefore, the electric discharge machining apparatus suffers from a difficulty that, when a mmachining operation is carried out with the electrode servo shaft 9 moving in a direction perpendicular to its axial line, the movement range of the shaft 9 is restricted due to the provision of the cover 10. In order to shield the electromagnetic waves with the cover, the cover must be designed so that it is kept at the same potential as the machining reservoir 1; that is, it is kept grounded at all times. In addition, the cover must be so designed as to prevent the leakage of electromagnetic waves from a member which is provided to open and close the cover 5. Consequently, the electric discharge machining apparatus is also disadvantageous in that the cover is unavoidably intricate in construction.